Known hysteresis brakes (DE 4 424 457 A1) are used as yam brakes in textile machines for example. The feature typical of such machines is a plurality of production units which are arranged in a row and run the same processing cycle in parallel. The individual yarn brakes are under central electrical control, in that current from a DC voltage source is applied to the magnet windings of the hysteresis brakes. The braking moment is provided exclusively by the magnetic flux created by the electromagnetic winding in the air-gap, which is dependent on the current applied to the winding at the time. If there is no current, there is no magnetic flux and the braking moment created by the previously known hysteresis brake would be equal to zero in this case. This means that the yarn has no tension when using the hysteresis brake as a yarn brake, should the current supply fail.
This can have fatal results in textile machines. The yarns of all spindles lose tension. If in ply twisting for example the yarn brake of the outer yarn lacks braking moment, yarn will be drawn out of the creel until the yarn is completely lacking tension. On account of the twist imparted by the twisting or ply twisting process, the yarns have an internal torsional moment. If there is no longer any yarn tension, this leads to the formation of loops. These can no longer be stretched out as the machine runs on. An unacceptable fault develops in the finished product. In order to avoid such faults, the textile machine has to be completely cleared down and set up again. Half-finished spools cannot be further processed as a rule. Also the remainders of the supply spools not yet processed create problems. As well as the high costs of unusable casualty spools of ply and supply material, high down times of the textile machines result. In order to counter these problems caused by loss of current, the textile machines are equipped with an emergency current supply from batteries. The demands of space, cost and servicing this emergency current supply are however substantial. Moreover the emergency current supply and thus the braking moment only lasts until the capacity of the batteries is exhausted which is normally the case after about 20 minutes.